Universal Serial Bus (USB) battery charging detection specification defines signaling over differential data lines to detect multiple kinds of charging devices connected to the differential data lines so that a Host (i.e., Host processor and/or platform) can provide charging current to the connected device or the connected device can request to charge its own battery (e.g., request charging current of 1.5 A) from the Host. Charging devices include, for example, a dedicated charger, dock, legacy host, or device (e.g., phone, camera, tablet, etc.), or proprietary charger.
However, charge detection signaling may happen at a very early stage (e.g., before cold boot of the Host) and also during dead battery conditions. During cold boot, Host side transceiver is completely turned off (i.e., no power supplies are available to the Host). In this case, the Host processor starts to observe back power current flowing from the charging device through the differential data lines to the Host transceiver (i.e., the battery of the charging device begins to deplete through a low impedance path from the charging device to the Host transceiver). This back power current may damage the devices of the Host transceiver. Also when the Host side battery is dead and a device (powered or self-powered) is connected to the Host, the Host processor starts to observe back power current flowing from the device through the differential data lines to the Host transceiver (i.e., the battery of the device begins to deplete through a low impedance path from the charging device to the Host transceiver). In such a case, the back power current may damage the devices of the Host.